The overall energy homeostasis of a mammalian system requires a high degree of regulation to ensure the availability of the appropriate substrate at the appropriate time. Plasma glucose levels rise during the post-prandial state, to return to pre-prandial levels within 2–3 hours. During these 2–3 hours, insulin promotes glucose uptake by skeletal muscle and adipose tissue and decreases the release of free fatty acids (FFA) from adipocytes, to ensure that the two substrates do not compete with each other. When plasma glucose levels fall, an elevation in plasma FFA is necessary to switch from glucose to fat utilization by the various tissues.
In individuals with insulin resistance, FFA levels do not fall in response to insulin, as they do in normal individuals, preventing the normal utilization of glucose by skeletal muscle, adipose and liver. Furthermore, there is a negative correlation between insulin sensitivity and plasma FFA levels.
Hormone-sensitive lipase (HSL) is an enzyme, expressed primarily in adipocytes, that catalyses the conversion of triglycerides to glycerol and fatty acids. It is through the regulation of this enzyme that the levels of circulating FFA are modulated. Insulin leads to the inactivation of HSL with a subsequent fall in plasma FFA levels during the post-prandial state, followed by the activation of the enzyme when the insulin concentration falls and catecholamines rise during the post-absorptive period. The activation of HSL leads to an increase in plasma FFA, as they become the main source of energy during fasting.
The activation-inactivation of HSL is primarily mediated through the cAMP-protein kinase A and AMP-dependent kinase pathways. There are compounds like nicotinic acid and its derivatives, that decrease the activation of HSL via these pathways and cause a decrease in lipolysis that leads to a reduction in the FFA levels. These drugs have a beneficial effect in the utilization of glucose and in the normalization of the excess triglyceride synthesis seen in patients with elevated FFA. However, since these pathways are used by other processes in the body, these drugs have severe side effects.
We have found compounds that specifically inhibit the lipolytic activity of HSL and lead to a decrease in plasma FFA levels. These compounds can be used to treat disorders where a decreased level of plasma FFA is desired, such as insulin resistance, syndrome X, dyslipidemia, abnormalities of lipoprotein metabolism.
One object of the present invention is to provide compounds and pharmaceutical compositions that inhibit the lipolytic activity of HSL. A further object is to provide compounds which have good pharmaceutical properties such as solubility, bioavailability etc.